This invention relates in general to ink jet drop generators and more particularly to an ink jet drop generator which continuously removes ink drops from its outer surface and which has features reducing fluidic crosstalk between emitters. There are a variety of ink jet printers and plotters which produce drops by different means including continuous-jet emitters, in which droplets are generated continuously at a constant rate under constant ink pressure, electrostatic emitters, and drop-on-demand emitters (or impulse jets). These emitters include means for producing a droplet, a nozzle to form the droplet, means for replacing the ejected ink and a power source to energize ejection of the droplet. The nozzles are used to control the shape, volume, and/or velocity of ejected droplets. Such devices employ either a single nozzle or a plurality of nozzles arranged in a linear or a planar pattern. All of these ink jet devices are subject to problems caused by wetting and contamination of the nozzles by ink and its residues on the outer surface of the nozzle.
Wetting of the outer surface of the ink jet nozzle can be caused by a variety of sources such as by droplets dislodged from the nozzles by shock or vibration. Ink spray produced during drop ejection can also deposit ink on the nozzle. Similarly, excess pressure in the ink in the ink reservoir or refill channels either during shipping, during operation or during the priming step in which air is bled from the channels connecting the emitters to the ink reservoir can force ink out of the nozzles onto the outer surfaces of the nozzles. Various types of malfunctions such as gas bubbles trapped in the nozzle can also cause ink to be deposited on the outer surface. The result of wetting the nozzle outer surface is usually a combination of fluid drops and dried ink residues which can prevent emission of ink droplets or disturb their trajectory and stability. In order to achieve high quality printing and/or plotting from an ink jet device, it is important that the nozzles remain free of obstructions and contamination and that the meniscus of the ink in each nozzle be predictable in its extent, orientation and location. For proper operation, surface fluid and residues must be prevented from accumulating on or near the nozzles.
Some previous solutions of the wetting problem have involved non-wetting surfaces and associated hardware and plumbing to remove and dispose of accumulated surface fluid. In these solutions, a non-wetting ink jet nozzle surface is utilized so that ink drops tend to bead up on the surface rather than adhering to and spreading out over the surface. When an ink drop reaches a critical size, its weight overcomes the attraction it has to the surface so that it either falls off of the surface or runs off of the surface without leaving a significant trail of ink. An external gutter typically collects such drops either for clean disposal or for return to the ink reservoir after being filtered to remove impurities. Such non-wetting surfaces limit the accumulation of ink on the outer surface of the ink jet nozzle but do not solve the problem of removal and disposal of ink and its residue from the outer surface.
In other previous solutions, various mechanical methods are used to clean the outer surface. Some of these methods include directing jets of air at the surface to blow away drops, wiping the surface or rolling an absorbant roller across the surface. This mechanical cleaning requires additional mechanisms, is inherently intermittent, introduces idle time for the ink jet device, and the wiper or roller can itself become a source of contamination. In particular, the intermittent nature of cleaning the nozzle surface may allow excessive surface fluid to accumulate, interfering with operation and allowing residues to form.
Another problem to which multiple emitter ink jets are susceptible is fluidic crosstalk between emitters. Linear and planar arrays of emitters often are connected by short refill channels to a common fluid-filled cavity, referred to as a plenum, which is in close proximity to the emitters and from which ink is drawn to refill the ink jet emitters after a droplet or droplets of ink are ejected. When ink is ejected from one emitter, a pressure disturbance is produced in the plenum which can disturb the ink in other nearby emitters. In addition, after ink has been ejected from an emitter, the flow of ink within the plenum to refill that emitter may disturb the ink in other nearby emitters.
In order to achieve high quality printing and plotting with ink jets, it is important that the ink within an emitter be substantially quiescent just before ink is ejected from that emitter. The ink forms a meniscus at the outer opening of each of the nozzles. These menisci can be caused to oscillate as a result of pressure waves and fluid flow in the vicinity of the emitter. If an emitter is caused to eject a droplet while its meniscus is oscillating, the size of the resulting droplet and its trajectory are essentially uncontrolled and can vary depending on the phase of this oscillation at the time of ejection. In severe cases, such disturbances can cause unwanted droplets to be emitted from one or more adjacent emitters. Therefore, it is important to reduce the disturbance of ink in an emitter caused by the ejection of ink from other emitters. In addition, oscillations of the meniscus in an emitter during its refill cycle should be well-damped such that a secondary droplet is not emitted and the fluid in the nozzle is quiescent for the next ejection cycle.